In the oil and gas exploration and production industry, a wellbore or borehole of an oil or gas well is typically drilled from surface to a first depth and lined with a steel casing. The casing is located in the wellbore extending from a wellhead provided at surface or seabed level, and is then cemented in place. Following testing and other downhole procedures, the borehole is extended to a second depth and a further section of smaller diameter casing is installed and cemented in place. This process is repeated as necessary until the borehole has been extended to a location where it intersects a producing formation. Alternatively, a final section of tubing known as a liner may be located in the wellbore, extending from the lowermost casing section or casing ‘shoe’ to the producing formation, and is also cemented in place. The well is then completed by locating a string of production tubing extending from surface through the casing/liner to the producing formation. Well fluids are then recovered to surface through the production tubing.
However, before the well can be completed and well fluids recovered to surface, it is necessary to clean the lined wellbore and replace the fluids present in the wellbore with a completion fluid such as brine. The cleaning process serves, inter alia, to remove solids adhered to the wall of the casing or liner; to circulate residual drilling mud and other fluids out of the wellbore; and to filter out solids present in the wellbore fluid. Much of the solids present in the wellbore are found on the surface of the casing/liner, and may be rust particles and metal chips or scrapings originating from equipment used in the well and from the casing/liner itself.
Various types of cleaning tools are known, including mechanical cleaning tools which physically wipe or scrap clean the surface of the casing/liner. One type of mechanical cleaning tool is generically referred to as a casing scraper. Casing scrapers typically incorporate scraper blades designed to scrape the inner surface of the casing/liner, for removing relatively large particles of debris from the surface of the tubing. Other types of mechanical cleaning tools incorporate brushes or other abrading elements or surfaces.
Whilst these mechanical cleaning tools have been shown to be effective in cleaning a wellbore, it is generally desired to improve the cleaning action of tools of this type.